In these sorts of applications, both "so" and "then" can be used interchangeably, and mean more or less the same thing.
Personally, to my ear, there is a slightly different nuance between them, but it is not anything very large or significant:
- "Then" has a little bit more of the feeling of a logical appeal. That is, it suggests "You should realize for yourself (if you think about it) that this follows from the previous thing"
- "So" has slightly more of the feeling of pushing your own decision on the other person. That is, it suggests "I have decided that this follows from the previous thing"
But many people may not even make this distinction themselves, and may not see any difference between them..